This invention relates to the preparation of paving materials, and specifically to an improved method and apparatus for preparing asphaltic concrete.
One method of preparing asphaltic concrete is to mix aggregate (e.g. limestone, gravel, granite, trap rock or similar materials) and liquid asphalt in a device known as a "drum mixer". A typical drum mixer comprises an elongated, hollow cylindrical, steel drum, the longitudinal axis of which is disposed at an inclination of approximately three degrees to six degrees with respect to the horizontal. The drum is supported for rotation about its longitudinal axis on rollers, and gear or chain drive means are provided to effect rotation. Aggregate in the desired size gradation is introduced into the upper end of the drum, and is slowly carried to the opposite end as a result of drum rotation. Flights are provided within the drum to lift the aggregate as the drum rotates, and to cause the aggregate to fall in a continuous shower across the drum's cross-section.
Air is drawn through the drum from the upper to the lower end, and passes out of the lower end through a stationary housing, and thence into a dust collector (typically a bag house) which has an exhauster at its downstream side.
A burner, located at the upper end of the drum, projects a flame into the drum which heats the air passing through the drum. The hot air removes moisture as it encounters the showering aggregate as it passes through the hot upper or drying section. Hot liquid asphalt is introduced into the drum through a supply pipe, having an outlet at an intermediate location in the drum, causing mixing in the lower and cooler section. Preferably a suitable shielding device is provided which shields the outlet of the asphalt supply pipe from the flame of the burner while allowing the dried aggregate to pass to a location where it can become mixed with the asphalt. As the drum rotates, the dried aggregate becomes mixed with the liquid asphalt to produce an asphaltic concrete product. The product is discharged from the lower or discharge end of the drum. The hot air flowing through the drum supplies heat to the mixture in the drum so that the product is discharged at the desired temperature, which is typically between about 270.degree. and 280.degree. F.
As the aggregate moves through the drying zone of the drum, dust is produced which is picked up by the air flowing through the drum. Some of the dust produced in the drying zone is captured by the asphalt being showered from the outlet of the asphalt supply pipe onto the aggregate in the mixing zone. The remaining dust which is not captured in the mixing zone is collected in the dust collector.
The capture of moderate quantities of fine dust by the liquid asphalt in the mixing zone is beneficial, since the fine dust serves as a filler, and since the capture of fine dust reduces the dust collection requirements. In certain localities, however, and notably in Kentucky and Tennessee, the available aggregates tend to produce excessive quantities of dust. If these aggregates are mixed with asphalt in a conventional drum mix plant, the product frequently fails to meet the gradation specifications for the final mix. Accordingly, where excessive dust conditions are encountered, instead of using a drum mixer, the usual practice has been to prepare asphaltic concrete by using a batch process in which drying of the aggregate is carried out in a separate drying drum, and mixing of the dried aggregate with asphalt takes place in a pug mill at a separate location. The bath process is well recognized as more expensive than the continuous drum mix process.
Several types of drying drums, for use in the batch process, are known in the art. The most popular drying drum is the conventional counterflow dryer in which aggregate and air flow through the drum in opposite directions. Dust is carried with air from the air outlet end to a dust collector. A burner is located at the aggregate outlet end (and air inlet end) and projects a flame into the drum in order to effect drying.
The conventional counterflow dryer is somewhat inefficient because of the fact that the aggregate, as it enters the drum, first encounters cool moist air leaving the drum, and only thereafter encounters warm dry air.
A parallel flow dryer, having the burner situated at the aggregate inlet/air inlet end overcomes this problem but presents aggregate temperature control problems that make it unpopular.
In order to meet the problem of inefficiency in the conventional counterflow dryer, and at the same time avoid the temperature control problem which would arise if the burner were remote from the aggregate outlet, a center draft dryer has been used. The center draft dryer comprises a drum having burners at both ends, and openings at a central location in communication with a collection housing through which air is drawn and carried to a dust collector. Air enters the center outlet dryer through both ends, and travels toward the center outlet where it is withdrawn along with dust. Aggregate travels from one end of the dryer to the other. At the side of the center outlet at which aggregate is introduced, aggregate and air move in the same direction. In this section, the differences between the temperatures and moisture contents of the air and aggregate are such as to promote efficient drying. The burner at the end of the dryer at which aggregate is withdrawn effects further drying of the aggregate, and also provides for better control of the temperature of the aggregare leaving the dryer.
It is also known to provide central openings in a mixing drum for the purpose of introducing used asphalt pavement for recycling.
Despite the fact that these two devices having central openings have been well known in the art for a long period of time, no one has recognized heretofore that the provision of central openings in a drum mixer can be used to provide a solution to the dust problem which has been throught to require use of a separte dryer and mixer.